This invention relates to a magnetic recording medium excellent in durability and a resin composition used therefor.
Magnetic discs are applied to a variety of fields of devices using large capacity information recording and reproducing devices. Typical examples are floppy discs and hard discs for digital signal recording devices. These magnetic discs are required to have larger recording capacity and reliability durable for a long period of operation. Magnetic discs are subjected to writing and reading out information while conducting an operation of sliding on a head. In a particular case, there is a device wherein a magnetic disc is not slided on a head but kept in a very narrow distance to write and read out information. Even in such a case, it is admitted that a head contacts with a magnetic disc by sliding in a certain probability. An increase of memory capacity inevitably requires severe sliding conditions, since the distance between the head and disc becomes narrower and the probability of contacting and sliding becomes higher. Thus, the improvement of durability is inevitable element. At present, there is no magnetic discs having sufficient durability.
In order to improve the durability, lubricants are used to reduce friction and wear at the time of sliding. According to a known method, a lubricant such as a long-chain fatty acid derivative, e.g. oleic acid, stearic acid, etc. is impregnated into a magnetic recording medium layer of a magnetic disc. This method is usually applied to magnetic recording media obtained by coating a mixture of magnetic powder and a binder. But the lubricating effects under severe sliding conditions cannot be expected sufficiently and thus it is difficult to expect lubricating properties for a long period of time.
In order to improve this, there are proposed to coat fluorine-containing lubricants more excellent in lubricating properties on magnetic disc surfaces (U.S. Pat. Nos. 3,490,946 and 3,778,308). According to these coating methods of lubricants, a great effect may be expected at the initial time, but the lubricants are gradually released from the disc surfaces to undesirably reduce the lubricating effect when used for a long period of time, due to high speed revolution of the magnetic disc.
On the other hand, in order to prevent the release of lubricants at initial time, there are proposed to coat fluorine-containing surface active agents enhanced in adsorption on the magnetic recording medium surfaces (Japanese Patent Unexamined Publication Nos. 59-116931, 58-41431, 58-29147, 57-154619, and 57-44226). The fluorine-containing surface active agents are, for example, compounds represented by the formula: EQU C.sub.n F.sub.2n+1 --X
wherein n is an integer of 4 to 13; X is a polar group such as --SO Me (Me=k or Na), --SO.sub.2 F, --COONH.sub.4, --COOH, --SO.sub.3 H, or --OH, as disclosed in Japanese Patent Unexamined Publication No. 58-29147 But since these fluorine-containing surface active agents are not fixed strongly on the surface, they are flown out from the sliding surface at the time of sliding, which results in not sufficiently improving durability of magnetic discs and also not improving durability of memory devices.
Further, in order to prevent the release of lubricants at initial time, there are proposed to fix the fluorine-containing lubricants to the surfaces of magnetic recording media using a compound of the formula: EQU C.sub.n F.sub.2n+1 --SiX.sub.3
wherein n is an integer of 4 to 13; X is a halogen, a nitrile group or an alkoxy group (U.S. Pat. No. 4,120,995, Japanese Patent Unexamined Publication Nos. 54-36171, 59-203239, 60-38730, 59-172159, and 61-39919). According to these methods, considerable durability can be expected since the lubricants are fixed to the magnetic recording medium surfaces by chemical reactions, but there are defects in that the reactions are difficult and it is very difficult to obtain uniform reacted films. In practical production processes, the application is very difficult. Further, since the fluorine chain length is too short, the lubricating effects are not so good, which results in not sufficiently improving the durability of magnetic recording media.
It is also proposed to use phosphorus-containing compounds of the formula: EQU Rf-P(O)-(OR).sub.m
wherein Rf is a perfluoroalkyl group; R is C.sub.Z H.sub.2Z+1 ; m is zero or an integer of 1 or 2; and Z is zero or an integer of 1 to 3 (Japanese Patent Unexamined Publication Nos. 60-109028, 60-101717, and 60-246020. These lubricants react with surfaces of metal films or oxidizing films of disc surfaces to be fixed. But since the fluorine-containing chains of these lubricating films are short, effects other than those of surface active agents can hardly be expected, and the durability is still insufficient.